1. Field of the Invention
The invention relates to the safety of lift cars or lift platforms used for lifting men and materials to high elevations. It relates particularly to a system for detecting speed in such cars or platforms and to braking means for stopping the platform when the speed exceeds a safe maximum.
2. Description of the Prior Art
The prior art devices for lifting men and materials to high elevations, where indoor type elevators are not available, include a variety of forms of lifts or platforms. These platforms usually include a broken cable mechanism which senses a loss of tension in a lift cable indicating that the cable has broken and operates a brake mechanism to lock the platform to guide cables or the like and thus prevent a dangerous fall. Examples of elevators having brake mechanisms to stop a fall in the event a hoist cable is broken are to be found in patents to R. E. Meyer, U.S. Pat. Nos. 3,517,774 and 3,517,775.
There exist situations in which the platform could descend at an excessive speed while the lift cable or cables remain intact and continue to exert appropriate tensile forces on the broken cable mechanism. Such a situation may arise if a winch is faulty, allowing the lift cable to pay out rapidly. Excessive speed might also be reached if the lift cable broke at a point removed a long way from the platform, providing tension due to the counter-weight effect of cable weight over the hoist sheave which thereby prevents the broken cable device from activating.